This invention relates to metal detectors using orthogonal coils and in particular, to a new and improved coil configuration, coil assembly fixture, and method of making same.
In one conventional form of metal detector, first and second coils are mounted with their axes at right angles to each other so as to minimize the coupling between them. In another conventional form the coils are coplanar. The present invention is directed to the right angle or orthogonal configuration.
In the orthogonal type detector, an a-c voltage is applied to one coil and the signal produced at the other coil is amplified and electronically processed to drive a signal indicator. When the unit is moved into an environment containing a metallic object, the object perturbs the magnetic field produced by the source coil and a desirable signal is coupled into the detector coil. For a magnetic type object, the magnetic field is distorted by providing a low reluctance path. For a non-magnetic, metallic object, the source magnetic field produces surface eddy currents in the object, which in turn create a flux field that alters the source flux field. Ideally, there should be zero transfer of energy or zero coupling between the coils where no metallic objects are present. The introduction of a metallic object into the field of the unit then produces an increase in output of the second or detector coil and an indication at the indicator unit. The degree and stability to which the coils can be de-coupled in the non-metallic environment influences the range of the unit and the complexity of the detector electronics.
While the invention is described and discussed herein as a metal detector, it will be realized that the device is equally suited for detection of any object which disturbs the magnetic field. This includes magnetic non-metallic materials and magnetic non-conducting materials, such as ferrites, and conducting non-magnetic materials and conducting non-metallic materials, such as carbon and some liquids. While the principal use presently intended for the invention is the detection of metal objects, it is not so limited.
While zero coupling between coils is desired, it is not possible to obtain this in actual practice. The physical size and shape of the coils, the variations occurring during coil manufacture, and the opposing effects of magnetic and capacitive coupling result in residual coupling between the coils when actual equipment is built. In addition, changes in environmental conditions, particularly temperature, cause some change in the residual coupling even if it can be made very low initially. Various attempts have been made in the past to achieve a minimum coupling, and many of these utilize a third coil with some form of external adjustment. U.S. Pat. No. 3,457,502 and the references of record therein are typical of the prior art devices.
It is an object of the present invention to provide a new and improved metal detector using two prefabricated coils which are assembled together to produce the finished unit while providing very low intercoil coupling. A further object is to provide such a detector which does not require external resistance, capacitance or inductance adjustment to obtain the null or minimum coupling condition. Other objects are to provide a fixture for assembly of the detector and to provide a method for producing the detector.